This invention relates to high power lasers for industrial applications and, more particularly, to collimating or focusing the beams from the number of laser sources needed to achieve a desired power density.
Because a diode laser source emits a beam which typically exhibits different degrees of divergence along different axes, a lens or series of lenses such as microlenses are typically used to help focus the emitted light. It should be understood that xe2x80x9cfocusingxe2x80x9d is used here in the sense which includes a range of effects that a lens may have on a diverging beam, from decreasing the divergence to collimation to convergence to a small spot. Generally, the emitting apertures of a laser diode are rectangular in shape with the long dimension having a size of typically hundreds of microns, while the short dimension is typically one micron in size. Diffraction effects cause the emerging radiation to diverge, with the divergence angle being inversely proportional to the size of the aperture. The short dimension of the aperture is comparable to the typical laser diode wavelength of approximately eight hundred nanometers; diffraction effects result in large beam divergence in this, the xe2x80x9cfast axisxe2x80x9d, direction which may be as high as seventy five degrees. The sign of the divergence angle is known as the numerical aperture (NA), the beam having a lower numerical aperture along the direction of the stripe than perpendicular to the stripe. Typical values would be 0.1 NA and 0.33 NA respectively. In the applications of concern here, the radiation must be focused at some distance from the laser diode and it is desirable to concentrate the beam diameter so as to maximize the power density at the point of focus.
To provide more power than can be obtained from a single solid state laser, several laser sources can be assembled into a laser xe2x80x9cbarxe2x80x9d. Koester U.S. Pat. No. 3,670,260; U.S. Pat. No. 4,185,891; and Sprague, et al U.S. Pat. No. 4,428,647 show various ways to compensate for the fact that laser bars emit spaced-apart beams. In particular, the ""891 and ""647 patents show the use of micro-lenses between the laser and an objective lens in which each micro-lens reduces the angle of divergence of a respective light beam leaving the emitting surface of the laser bar. Reducing the divergence angle of the beams allows the objective lens to reduce the beam spacing to a degree that is substantially less than the set spacing between the beams at the laser bar.
While the use of a microlens array reduces the divergence angles of the individual beams emanating from the emitting sources of a diode laser, a laser bar which incorporates several, transversely separated diode lasers requires that an objective lens having a large numerical aperture be used if the beam is to be concentrated into a usefully small spot. Large numerical aperture objective lenses tend to be expensive. The problem becomes even more difficult when more power is required than can be delivered by a single laser bar. If it is attempted to use two laser bars, the lateral separation of their beams adds to the difficulty. It would be extremely advantageous to be able to focus such widely separated laser beams to an acceptable spot size without requiring the use of an expensive objective lens.
In the copending application of G. Treusch, a stack of laser beams emitted from a pair of laterally separated stack of laser diodes bars are focused into a single, vertical plane of stacked laser beams through the use of an interleaved array of angular glass plates. The array of glass plates presents complementary oblique entrance surfaces to the laser beams from successive levels of the laser bars. Each of the glass plates presents an exit surface parallel to its entrance surface, the parallel entrance and exit surface of said plates being spaced apart a sufficient distance to refract said laser beams so as to substantially overcome lateral displacement among the laser beams.
While the aforementioned application of G. Treusch eases the focusing requirements imposed on the objective lens by causing the laser beams from laterally separated stacks of laser bars to lie in a single vertical plane, not enough power may be available from the two stacks to achieve a desired power density in the beam that can be focused by a lens having a given numerical aperture. It would be extremely advantageous to be able to increase the power density provided from one or more stacks of laser diode bars.
In accordance with the principles of the present invention, in one illustrative embodiment, a greater power density is obtained by combining the beams from two different wavelength laser diode bars in a stack by using a compound prism having a dichroic portion that reflects one wavelength but passes the other. A group of diode lasers emitting beams at a first wavelength is positioned above and below a stack of laser diodes emitting beams at a second wavelength. The first wavelength beams are directed to a dichroic portion of the compound prism and are reflected outwardly. The second wavelength beams are directed to the opposite surface of the dichroic portion but pass through without reflection and so are directed into substantially the same outward plane as the first wavelength beams. Accordingly, the beams of two different wavelengths are combined in the outward direction to achieve a greater power density in the given plane than can be obtained with beams of either wavelength alone. Advantageously, a second stack of dual wavelength laser beams may be concentrated with a second compound prism and the two stacks of laser beams may be combined using the glass plate technique disclosed in the above-mentioned copending application of G. Treusch.